1. FIELD OF THE INVENTION
The present invention relates to a power source for arc welding and more particularly to an inverter controlled-type power source for arc welding which is suited for AC TIG welding purposes and capable of suppressing any excessively large transient voltage and stably and reliably effecting the polarity reversal of an output current.
2. DESCRIPTION OF THE RELATED ART
Recently, AC-DC power sources for arc welding of the type reduced in size and weight through the use of inverter control have been put in practical use. In the case of AC TIG welding, however, during the current reversal, particularly when the polarity of the current flowing between the electrode and the base metal is reversed from one polarity where the electrode side is negative (positive polarity) to the other polarity where the electrode side is positive (negative polarity), there is a tendency for the arc to be extinguished. Thus, there have been used arc welding power sources of the type including a reignition aiding circuit for applying a high-frequency high voltage across the electrode and the base metal to prevent the occurrence of arc extinction. FIG. 1 of the accompanying drawings shows the circuit construction of this type of power source for arc welding.
In FIG. 1, after the AC voltage applied to an input terminal 1 from a commercial power supply has been converted to a DC voltage by a rectifier 2, the DC voltage is converted to a high-frequency AC voltage (e.g., 20 kHz) and applied to a main transformer 4 through an input converter 3. After the AC voltage has been reduced to a suitable voltage for welding purposes by the main transformer 4, the reduced voltage is applied through a current detector 5 to an output rectifier 6 to produce a DC voltage, which is smoothed by a reactor 7 and a capacitor 8. The DC voltage is converted again to an AC voltage by an output inverter 9 and then supplied to an arc load (not shown) through an output terminal 10.
In this case, a current control circuit 13 determines a desired output current based on a preset value of the output current inputted from an output current setting means 14, the desired output current is applied to an error amplifier 12 which compares the desired output current with the actual output current detected by a current detector 5 thereby producing an error signal between them after being amplified. The amplified error signal is applied to a pulse width control circuit 11 which controls the pulse width of the input inverter 3 so as to reduce the error signal substantially to zero. On the other hand, a rectangular wave generator 16 generates a reference rectangular signal having a repetition frequency and a duty cycle which are respectively corresponding to the preset values of reversing frequency setting means 17 and positive polarity negative polarity ratio setting means 18 so that in response to each of the leading and trailing edges of the reference rectangular signal the polarity of the output current is reversed by the output inverter 9 through a drive circuit 24. This relation is shown in FIG. 2. Note that the output inverter 9 includes transistors T.sub.1 to T.sub.4 and diodes D.sub.1 to D.sub.4 which are connected in bridge form as shown in FIG. 3 so that an AC output is generated by turning on and off alternately the two paired transistors T.sub.1, T.sub.2 and T.sub.3, T.sub.4 in synchronism with the leading and trailing edges of the reference rectangular signal as mentioned previously and a DC output is generated by continuously turning on either one of the two transistor pairs while maintaining the output of the rectangular wave generator 16 at "0" or "1".
In FIG. 1, if an arc cannot be reignited only by the voltage supplied from the main transformer 4 during the polarity reversal of the output current, a transient voltage is produced in the reactor 7 of the output circuit due to a rapid decrease in the load current. The magnitude of this transient voltage is determined by the inductance value of the reactor 7 and the value of the current flowing therein during the polarity reversion. With the circuitry of FIG. 1, the current value during the polarity reversal varies for example over a wide range from 300 A to less than 100 A and the transient voltage produced in the reactor 7 during the polarity reversion also varies correspondingly over a wide range. Thus, a capacitor 8 of a large capacity is required to absorb any excessively large transient voltage to prevent the transistors, etc., of the output inverter 9 from being destructed. Also a reignition aiding circuit 25 of a large capacity is required to supply the required voltage (about over 200 V) for the reignition of an arc when the transient voltage is so small that the voltage on the capacitor 8 does not reach the reignition voltage, thereby increasing the cost of the device. Moreover, there are instances where the inductance value of the reactor 7 is limited to avoid the generation of an excessively large transient voltage thus failing to fully smooth the current ripple and where the increased capacity of the capacitor 8 deteriorates the output control response.
FIG. 3 shows an example of the reignition aiding circuit 25. The circuit of FIG. 3 is designed, by taking into consideration the fact that the AC TIG welding power source of the inverter-controlled type produces a relatively low reignition voltage of about 200 V during the current reversal and that the high frequency superposing method tends to cause radio interference, etc. A high DC voltage (about 200 V) is applied during the current reversal for superimposition on the secondary-side main circuit of the welding power source thereby maintaining the arc. The circuit of FIG. 3 receives a high-frequency AC power from the primary side of the transformer 4 in FIG. 1 and a high DC voltage of about 200 V is produced through a transformer 26, a rectifier 28 and a smoothing capacitor 30. The ends of the capacitor 30 are connected across the output lines of the output rectifier 9 in the welding power source so that when the output current of the output inverter 9 is reversed to reignite an arc, the high DC voltage is applied across an electrode 10 and base metals 34 to maintain the arc.
While the reignition aiding circuit 25 is effective for the stabilization of the AC arc, the current continues to flow in a resistor 32 even after the reignition of the arc causing a voltage drop with the result that it is necessary to increase the current capacity of the components forming a high voltage superposing circuit, e.g., the transformer 26, the rectifier 28, the smoothing capacitor 30 and the resistor 32 and these components are increased in size and weight, thereby ruining the merit of decreasing the size and weight of the welding power source owing to the inverter control and also deteriorating the efficiency.
Also, where the welding is effected with a large current, during the current reversal of the output inverter 9 a spike voltage is caused by the inductance of the main circuit thereby destroying the elements such as the transistors.